A. Technical Field
The present invention relates generally to optical networking node architectures and configurations, and more particularly, to protection switching of FEC-encoded signals.
B. Background of the Invention
The importance of optical networking technology in today's society is well understood. Optical networks allow large amounts of information to be transmitted at high data rates across very long distances. In optical long-haul or metro-ring scenarios, multiple channels or wavelengths are multiplexed together and inserted into a fiber optic cable that spans a relatively long distance. The optical signal, comprising multiple wavelengths, propagates within the fiber optic cable until its destination is reached. This signal is demultiplexed and the individual wavelengths further processed at a destination node.
The reliability of networks is extremely important and network operators require that their networks operate with an extremely low failure rate. A network failure event may lead to a large amount of data being lost and may significantly impact the companies that rely on the network. These network failures may also cause financial losses to the network providers and require significant efforts to repair. In order to minimize the number of failure events, networks are built with layers of redundancy.
Networks generally provide redundant paths between network nodes to improve reliability. Data is switched between these paths in the events of failures, latency issues or other factors that may reduce the reliability of data be transmitted between the nodes. The management of traffic across these redundant paths may be performed using different techniques and in accordance with various protocols.
FIG. 1 illustrates an exemplary wave division multiplexing (“WDM”) network link. The link includes a plurality of transmitters 110, each having a forward error correction (“FEC”) encoder 112 and electrical-to-optical converter 115. A multiplexer 120 multiplexes the optical signals generated from the transmitters 110 into a WDM signal that is inserted into optical fiber. The link may also include various intermediary devices such as optical amplifiers 130 or regenerators (not shown) to amplify or otherwise repair the WDM signal as it propagates along the link. At a receiver node, the WDM signal is demultiplexed into wavelength components by a demultiplexer 140. These wavelengths are sent to a plurality of receivers 150, each having an optical-to-electrical converter 155 and FEC decoder 157.
WDM and other metropolitan, regional and long haul fiber optic transmission systems typically utilize forward error correction (“FEC”) encoding and decoding to correct single or multiple bit errors in a transmitted bitstream. The encoding and decoding process is typically limited to one transmitter/receiver link, defined as being the signal path between the electrical-optical converter 115 of the transmitter 110 and the optical-electrical converter 155 of the receiver 150. This FEC encoding compensates for errors generated within the optical signal and also provides an indication of the health of the link or signal path. However, as will be discussed in more detail below, FEC encoding may also mask indications of a degrading link, optical path or span therein.
FIG. 2 illustrates an exemplary protected network in which multiple redundant paths are provided in order to provide high reliability against fiber and equipment failures. A client signal is provided to a multiplexer 210 that is coupled to a working path 220 and a plurality of protection paths including a first protection path 230 up to an Nth protection path 240. Each path, including the working path and protection paths 1 through n, can include one or more concatenated transmitter/receiver links (link a through span m) and may include additional optical or electrical switches (not shown).
The client signal is “bridged” or replicated to the multiple independent paths, one of such paths being selected to be delivered to the client receiver. Each of the paths terminates at a receiver 240 that includes an end-to-end error detection module 250. A selector 270 selects a signal from one of the paths and transmits the signal to a client receiver. A detected error statistics memory 260 may also be provided that stores information about identified errors on each of the signal paths.
In selecting a desired signal path from multiple paths of transmissions of the same client signal, the decision of selecting the optimum client signal path is a determination based upon schemes that select the signal path according to various factors. A first factor that may be used is the absence of a recognizable signal generally indicated by a loss of optical power or a loss of frame condition, which may be detected at the end terminal receiver of the signal selector 270. A second factor is recognizable signal with an unacceptable bit error rate at the end terminal receiver of the signal selector 270.
A third factor is the reception of an alarm indication signal (“AIS”) which is an indication generated by an upstream node and transmitted to the far end receive node. FIG. 3 illustrates an exemplary AIS scenario in which a signal is transmitted by a transmitter 310 on a link comprising multiple intermediary nodes 320, 330, 340. An end receiver 340 receives the client signal and an AIS detector 350 identifies the presence of the AIS. A selector 360 selects a particular client signal and forwards it onto a client receiver.
Referring to FIG. 3, the upstream intermediary node 320 detects an optical or digital impairment in the link and generates an AIS. The MS is inserted into the client signal and a transmitter transmits the AIS and client signal onto the link. The AIS may be inserted within the client signal or may have a dedicated channel in which MS and other control signals are communicated. After the AIS detector 350 receives the inserted MS, it may respond in a number of ways including switching the signal onto a redundant path.
A problem with this selection schemes is that, due to the nature of a FEC encoded system, the selection of a new desired signal path from among multiple signal paths often results in the transmission of a substantial number of errors to the client receiver before the selection can be transpired. More specifically, systems with high FEC gain will allow the client signal to be received essentially without errors even though the condition of the link over which the client signal is transported has a marginal condition in one or more of the concatenated spans that make up the signal path. By marginal condition, it is meant that at least one span has a marginal capacity to successfully transport the client signal but since the signal is FEC encoded, accurate recovery of the signal is realizable.
For example, the pre-FEC error rate may be marginally acceptable and then a subsequent change in the polarization mode dispersion causes the pre-FEC error rate to move from the marginally acceptable point to a point just below an acceptable rate, which will result in a high post-FEC error rate and loss of signal integrity. The far end selector may be able to select at this point a different signal path for the client signal reception, but due to the latency of the error detector in the receiver and the selector, at least a portion of the degraded or corrupted client signal may be transmitted to the client before the switching to a better client signal path of the client signal can be successfully transpired.